Calcium hardness (CH)
Also called: CH, calcium.
Calcium hardness (CH) is the amount of dissolved calcium in pool water. Plaster, gunite and pebble surfaces are calcium, so the water needs enough dissolved calcium that it isn't pulling more out of the finish, while too much calcium falls out as scale on tile, heaters and salt cells.
Why it matters
On vinyl or fiberglass the wall sets no calcium requirement, but the same water runs through your heater and salt cell, so most owners still hold some calcium rather than run it near zero. Raise it with calcium chloride, and start low, because no bottle brings calcium back down. The only fix for high calcium is dilution, a partial drain and refill. Evaporation leaves calcium behind, so hard fill water concentrates over a summer, and cal-hypo shock adds more. Judge the number through the saturation index, not by itself.